The present invention relates to a device and method for ensuring that the amplitude of signals fall within a predetermined range.
In particular, but not exclusively, the device and method can be used in a receiver for a wireless telecommunication network.
FIG. 1 illustrates a known wireless telecommunication network 2. The area covered by the network 2 is divided into a number of cells 4. Each cell 4 has associated therewith a base transceiver station 6. Each base transceiver station 6 is arranged to communicate with the terminals 8 located in the cell 4 associated with that base transceiver station 6. The terminals 8 may be mobile stations which are able to move between the cells 4.
Each base transceiver station is, in the GSM standard (Global System for Mobile Communications), arranged to receive N channels out of M available channels C1 . . . CM, as illustrated in FIG. 2a. The M channels C1 . . . CM occupy a bandwidth of XMHz. Each channel therefore has a width of X/M MHz. Typically this will be around 200 KHz. Each channel is divided into frames F one of which is shown in FIG. 2b. Each frame is divided into eight slots S1 . . . S8. The GSM standard is a time division multiple access (TDMA) system and accordingly different mobile stations will be allocated different slots. Thus, the base transceiver station will receive signals from different mobile stations in different time slots in the same channel. N is usually much less than M.
There are two forms of GSM, E_GSM and GSM1800. E_GSM operates in the frequency band of 880-915 MHz for the receipt of signals by the base station. GSM 1800 operates in the frequency band of 1710 to 1785 MHz for the receipt of signals by the base station. E_GSM 900 and GSMxe2x80x941800 operate with bandwidths of 35 MHz and 75 MHz respectively. For E_GSM M=125 and for GSM 1800 M=375.
Reference is made to FIG. 3 which shows part of a known base transceiver station 9 which is arranged to receive N channels at the same time. For clarity, only the receiving part of the base transceiver station 9 is shown. The base transceiver station 9 has an antenna 10 which is arranged to receive signals from mobile stations in the cell served by the base transceiver station 9. The base transceiver station comprises N receivers R1, R2 . . . RN. Thus one receiver is provided for each channel which is to be received by the base station 9. All of the receivers R1-RN are the same and accordingly the components of the first receiver R1 only are shown.
The first receiver R1 comprises a first bandpass filter 12 which is arranged to filter out signals which fall outside the bandwidth in which the M available channels are located. The filtered output is input to a first low noise amplifier 14 which amplifies the received signals. The amplified signal is then passed through a second bandpass filter 16 which attenuates spurious frequencies, noise, and harmonics or the like introduced by the first amplifier 14. The output of the second bandpass filter is connected to a mixer 18 which receives a second input from a local oscillator 20. The frequency of the output of the local oscillator 20 will depend on the frequency of the channel allocated to the particular receiver. The output of the second bandpass filter 16 is mixed with the output of the local oscillator 20 to provide a signal at an intermediate frequency IF, which is less than the radio frequency at which the signals are received. The intermediate frequency IF output by the mixer 18 of each receiver will be the same for all receivers.
The output of the mixer 18 is input to a third bandpass filter 22 which filters out spurious signals and the like generated by the mixer 18. The output of the third bandpass filter 22 is amplified by a second amplifier 24 and output to a surface acoustic wave (SAW) filter 26. The surface acoustic wave filter 26 filters the adjacent signals and interfering signals within the bandwidth X except that of the channel allocated to that particular receiver. In other words, all the channels received by the antenna 10 with the exception of the channel allocated to the receiver will be filtered by a combination of the first to third bandpass filters and the surface acoustic wave filter 26. The output of the surface acoustic wave filter 26 is connected to an automatic gain control unit 28 which alters the gain of the signal so that it falls within the dynamic range of an analogue to digital converter 30.
If the amplitude of the signal input to the analogue to digital converter 30 is too high, then the converter 30 will become saturated, giving rise to phase errors, recovery time problems and unwanted noise. By the same token, if the signal input to the analogue to digital converter 30 is too low, then the received signal may be below the noise floor of the converter 30. In other words, if the signal is too small, it will be swamped by the noise floor and information carried by the signal may be lost.
One problem with the known architecture is that it is necessary to provide a receiver for each channel. This is to ensure that each signal which is input to an analogue to digital converter is within the dynamic range of that converter. The need to provide a receiver for each channel increases substantially the costs of the base transceiver station. It is therefore an aim of embodiments of the present invention to solve or at least mitigate this problem.
Another problem with the known receiver is that it is necessary to use a SAW filter to filter out the adjacent channels and high power interferers which compromise the dynamic range of the converter 30. SAW filters are expensive. It is therefore also an aim of embodiments of the present invention to provide a device for ensuring that signals fall within the dynamic range of, for example, an analogue to digital converter. It is preferred that this device not require the use of SAW filters.
According to one aspect of the present invention, there is provided a device for ensuring that the amplitude of signals fall within a predetermined range, said device comprising input means for receiving a plurality of input signals at substantially the same time; a first path for increasing the amplitude of any of the input signals having an amplitude below a first threshold; a second path for decreasing the amplitude of any of the input signals having an amplitude which exceeds a second threshold; and combining the outputs of the first and second paths to provide the plurality of signals having amplitudes between said first and second thresholds.
Thus, it can be ensured that signals which are too large are reduced to a lower amplitude whilst signals which are too small are increased to a larger amplitude. In this way, the device is able to ensure that the amplitude of signals fall within a predetermined range.
The first and second thresholds may be the same or different.
Preferably, the second path comprises attenuator means. Preferably, signals which have an amplitude below the second threshold are removed by the second path. Those signals may be removed by reducing those signals to substantially zero. This can, for example, be achieved by the attenuator means and is both simple and cost effective.
Preferably, the first path comprises amplifier means.
Removing means may be provided to remove signals from said first path having an amplitude exceeding said first threshold. An output of the second path may be introduced in the first path and the output of the second path may be used by said removing means to cancel out the signals in the first path having an amplitude exceeding the first threshold. A phase shift may be provided so that one of the signals output by the second path introduced into the first path and signals on the first path is 180xc2x0 out of phase with respect to the other of the signals output by the second path introduced into the first path and the signals of the first path. Preferably, the removing means comprises a summer for summing the input to the first path with the output from the second path to cancel signals which have an amplitude which is greater than the first threshold. This is particularly advantageous when the second path provides signals which initially exceed the second threshold and the first path provides signals which initially have an amplitude below the first threshold.
The input means may be connected to splitter means which provide a plurality of sets of signals, each set comprising a plurality of input signals. One set may be input to the first path, the other set may be input to the second path. Preferably, the first amplitude of the sets of signals are the same.
Preferably, the device has output means connected to the output of the first and second paths for outputting said signals to an analogue to digital converter. A device as described hereinbefore may be provided in combination with an analogue to digital converter. A receiver may incorporate a device as described hereinbefore. That receiver may be incorporated in a base transceiver station.
According to a second aspect of the present invention, there is provided a method for ensuring that the amplitude of signals fall within a predetermined range, said method comprising the steps of receiving a plurality of input signals at substantially the same time; increasing the amplitude of signals having an amplitude below a first threshold; decreasing the amplitude of signals having an amplitude which exceeds a second threshold; and combining the said signals thus providing a plurality of signals having amplitudes between said first and second thresholds.